Transcript ch08

Chapter 8
Graphics and Color
Principles of Web Design, Third Edition
Objectives
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Understand graphics file formats
Choose a graphics tool
Use the <img> element
Control image properties with CSS
Understand computer color basics
Control color properties with CSS
Work with images and color
Control background images with CSS
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Understanding Graphic Files Formats
• You can currently use only three image file
formats on the Web: GIF, JPG, and PNG. A new
format, SVG, is not yet in common use.
• These formats all compress images to create
smaller files. Knowing which file format to use for
which type of image is important.
• If you choose the wrong file type, your image
won’t compress or display properly
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GIF
• GIF uses a lossless compression technique,
meaning that no color information is
discarded when the image is compressed
• The color depth of GIF is 8-bit, allowing a
palette of no more than 256 colors
• GIF excels at compressing and displaying flat
color areas, making it the logical choice for
line art and color graphics
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GIF Transparency
• With GIF files, you can choose any one color
in an image to appear as transparent in the
browser
• The background color or pattern will show
through the areas that you have designated
as transparent
• Using transparent areas allows you to create
graphics that appear to have an irregular
outside shape, rather than being bounded by
a rectangle
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GIF Animation
• The GIF format lets you store multiple images
and timing information about the images in a
single file
• This means that you can build animations
consisting of multiple static images that play
continuously, creating the illusion of motion
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JPG
• JPG is best for photographs or continuous
tone images
• JPGs are 24-bit RGB images that allow
millions of colors
• JPGs use a “lossy” compression routine
especially designed for photographic images.
When the image is compressed, some color
information is discarded, resulting in a loss of
quality from the original image.
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JPG
• When you create the JPG file, you can also
manually balance the amount of compression
versus the resulting image quality
• The higher the compression, the lower the
image quality. You can play with this setting to
create files that are as small as possible but
still look good.
• Many photos can sustain quite a bit of
compression while still maintaining image
integrity
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PNG
• A royalty-free file format that is intended to
replace GIF
• This lossless format compresses 8-bit images
to smaller file sizes than GIF
• PNG supports transparency and interlacing
but not animation
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SVG
• A new standard from the W3C
• A language for describing two-dimensional
graphics using XML
• SVG graphics are scalable to different display
resolutions and are printable
• Not yet supported by most browsers
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Using Interlacing &
Progressive Display
• Most Web-capable graphics editors let you
save images in an interlaced or progressive
format
• You can choose this display option when
creating GIF, PNG, and JPG files
• GIF and PNG files use interlacing, while
JPGs use progression
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Using Interlacing & Progressive
Display
• Interlacing and progressive display are
generally the same thing—the gradual display
of a graphic in a series of passes as the data
arrives in the browser
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Where You Can Find Images
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Stock photo collections
Digital cameras
Scanner
Public-domain Web sites
Clip art
Create your own
Remember to respect copyright laws!
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Choosing the Right Format
• GIF: The everyday file format for all types of
simple colored graphics and line art. Use GIF
sparingly for its animation capabilities to add
visual interest to your pages. GIF’s
transparency feature lets you seamlessly
integrate graphics into your Web site.
• JPG: Use JPG for all 24-bit full color
photographic images, as well as more
complicated graphics that contain color
gradients, shadows, and feathering
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Choosing the Right Format
• PNG: If the browsers are supporting it, use
PNG as a substitute for GIF.
• Because PNG doesn’t compress your 24-bit
images as well as JPG, don’t use it for
photos.
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Choosing a Graphics Tool
You use graphics software to create or
manipulate graphics.
Photoshop Elements ~ $50
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Using the <img> element
• By definition, <img> is a replaced element,
meaning that the browser replaces the <img>
element with the image file referenced in the
SRC attribute
• <img> is an empty element, so never use a
closing tag with it
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Using the <img> element
• The browser treats the image as it treats a
character: normal image alignment is to the
baseline of the text.
• Images that are within a line of text must have
spaces on both sides, or the text will touch
the image.
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<img> Element Attributes
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Replacing Img Attributes with Style
Sheet Properties
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Specifying alt and title Attribute Text
• The alt text is displayed if the image does not
appear, providing a description of the image
• The title text appears as a pop-up when the
user places the cursor over the image
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Specifying Image Width and Height
• Every <img> element on your site should
contain width and height attributes
• These attributes provide important
information to the browser by specifying the
amount of space to reserve for the image
• This information dramatically affects the way
your pages download to the user, especially
at slower connection speeds
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Controlling Image Properties with CSS
• Removing the hypertext border
• Aligning text and images
• Floating images
• Adding white space around images
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Removing the Hypertext Border
• When you create a hypertext image, the
browser’s default behavior is to display the
hypertext border around the image
• This border is often unnecessary as users
often use their mouse to point to each image
to see if the hypertext cursor displays
<img src=”globe1.gif” width=”100”
height=”100” alt=”globe” style=”border:
none” />
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Aligning Text and Images
• You can align text along an image border
using the align attribute
• Text and image alignment defaults to bottom
alignment, which means the bottom of the
text aligns with the bottom edge of the image
• Valid values are: top, middle, bottom, left,
right
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• <img src=”cycle.gif” style=”vertical-align: top” border=”1” />
• <img src=”cycle.gif” style=”vertical-align: middle”
border=”1”/>
• <img src=”cycle.gif” border=”1” />
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Floating Images
• The float property can be used to float an
image to the left or right of text.
• The following style rules create two classes of
<img> elements, one of which floats to the left
of text; the other floats to the right.
img.left {float:left;}
img.right {float:right;}
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<img src=”sample.gif” class=”left” />
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Adding White Space Around Images
• Add white space around your images to
reduce clutter and improve readability.
– As shown in Figure 8-15, the default spacing
is very close to the image.
• Use the CSS margin property to increase the
white space around an image.
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Adding White Space Around Images
The following style rule adds 15 pixels of white
space on all four sides of an image.
<img alt=”sailboat” border=”0” style=”margin: 15px;
float: left” src=”sail.gif” />
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Understanding Computer
Color Basics
• Monitors display colors by mixing three basic
colors of light: Red, Green, and Blue
– Intensity ranges from:
• 0% (complete absence of color) to 100% (complete presence
of color)
• Color depth
– Amount of data used to create the color
• 8-bit (256 colors), 16-bit and 24-bit (16.7M colors)
• Browser-safe palette (216 colors)
– Displays properly on Win & Mac at lowest depth
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Color Depth
• The amount of data used to create color on a
display is called the color depth
• If your users have a 24-bit color display, they
can appreciate the full-color depth of your
images. But many monitors cannot display
24-bit images.
• If your monitor doesn’t support the full color
depth of an image, the browser must resort to
mixing colors that attempt to match the
original colors in the image
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Specifying CSS Color Values
• Color names
• RGB color values
• Hexadecimal color values
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Using Color Names
• Sets color values using common color
names
– Aqua, Fuchsia, Lime, Red, etc.
• Limited to small range of colors
• Not a very specific representation of color
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Using RGB Color Values
• Numerical values that specify the blending of the
Red, Green, and Blue color channels
• Range: 0-100% (zero color to max color)
– Also: 0-255 (integer)
• Can be expressed as percentage or integer:
– P {color: rgb(0, 100%, 100%);}
or
– P {color: rgb(0, 255, 255);}
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Using Hexadecimal Color Values
• Numerical values that specify the blending of the
Red, Green, and Blue color channels
– Base 16 number system (0-9, A-F)
• Range: 00-FF (zero color to max color)
– Example: Red  FF 00 00
– The following rules specify the same color:
• P {color: #00FFFF;}
• P {color: rgb(0, 100%, 100%);}
• P {color: rgb(0, 255, 255);}
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Understanding Element Layers
• Background color layer—The backmost layer,
specified by the background-color property
• Background image layer—The middle layer,
specified by the background-image property
• Content layer—The frontmost layer; this is the
color of the text content; specified by the color
property
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Controlling Color Properties with CSS
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Specifying color values
Setting default text color
Changing link colors
Specifying background color
Setting the page background color
Creating a text reverse
Using background color in tables
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Specifying Color Values
The following style rules show the different methods of
specifying a color:
p {color: blue;}
/* color name */
p {color: #0000ff;}
/* hexadecimal value
*/
p {color: rgb(0,0,255);}
/* RGB numbers */
p {color: rgb(0%,0%,100%);} /* RGB
percentages */
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Changing Link Colors
• You can change the colors of hypertext links by
using the following special CSS classes.
• link—The unvisited link color; the default is blue.
• active—The active link color. This is the color
displayed when the user points to a link and
holds down the mouse button. The default is red.
• visited—The visited link color; the default is
purple.
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Changing Link Colors
You can use these special classes only with the
<a> tag.The syntax uses a colon (:) flag character
as shown in the following examples:
a:link {color: #000000;} /* new links are black */
a:active {color: #FF0000;} /* active links are red */
a:visited {color: #CCCCCC;} /* visited links are gray */
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Specifying Background Colors
• Background-color
– Sets the background color of any element
on a Web page (including padding area)
– By default, background color of any element
is transparent
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Specifying Background Colors
• Background-color
– Setting the Page Background Color
• Use body as the selector
body {background-color: ccc;}
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Creating a Text Reverse
• The background and foreground colors are
reversed
• The following rule sets the text color to white
and the background color to gray
h1 {color: fff; background-color:
gray; padding: .25em;}
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Specifying the Background
Image URL
Allows addition of a background image to an
entire Web page or to a single element
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Creating an Element Background
• Images can be applied to background of any
element.
• The following rule applies an image to the
background of the H1 element:
h1 {background-image: url(bluetex.jpg);
padding: .25em;}
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Specifying Background Repeat
Controls tiling of background images
body {background-image:
url(grayivy.jpg); backgroundrepeat: repeat-y;
}
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Creating a Vertical Repeat
• Allows creation of a vertically repeating
background graphic
body {background-image:
url(grayivy.jpg); backgroundrepeat: repeat-y;}
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Creating a Horizontal Repeat
• Allows creation of a horizontally
repeating background graphic
body {background-image:
url(grayivy.jpg); backgroundrepeat: repeat-x;}
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Creating a Nonrepeating
Background Image
• Allows creation of a single instance of an
image in the background
• The following style rule shows the use of
the no-repeat value:
body {background-image:
url(grayivy.jpg); backgroundrepeat: no-repeat;
}
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Specifying Background Position
• The background-position property lets you use
three types of values: percentage, length, or
keywords. Table 8-7 lists the values and their
meanings. Figure 8-34 shows the keyword
positions in the element box and their equivalent
percentage values.
body {background-image:
url(grayivy.jpg); background-repeat:
repeat-y; background-position: center;}
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body {background-image: url(lgivy.jpg);
background-repeat: no-repeat;
background-position: center;
}
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Positioning Vertical and Horizontal
Background Images
• Positions images that repeat on either the
horizontal or vertical axis of the web page
• The following rule positions the vertical
repeating background image along the right
side of the browser window:
body {background-image:
url(grayivy.jpg); backgroundrepeat: repeat-y; backgroundposition: right;}
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body {background-image: url(grayivy.jpg);
background-repeat: repeat-x;
background-position: bottom;
}
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Summary
• You currently can use only three image file
formats on the Web: GIF, JPG, and PNG.
– These formats all compress images to create
smaller files. Unless you choose the
appropriate file format, your image will not
compress and appear as you expect.
– SVG is a new file format from the W3C that
offers vector-based graphics for the Web.
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Summary
• Your computer monitor displays color by
mixing the three basic colors of light: red,
green, and blue (RGB).
– Colors vary widely from one monitor to another,
based on both the user’s preferences and the
exact brand of equipment.
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Most monitors have a resolution of 72
dpi.When creating, scanning, or importing
images, always change the final resolution to
72 dpi.
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Summary
• Reduce image size to the appropriate dimensions for
a Web page. If you must use a larger image, let the
user view a thumbnail first, and provide the file size
information.
• Test your colors carefully to make sure that the widest
variety of users can access your content. Consider
restricting your color palette to the colors available in
the browser-safe palette to ensure the greatest
portability of your Web pages.
• Color names are not always the best way to specify
color values because of their variable nature.
Consider using RGB or the more common
hexadecimal values instead.
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Summary
• Use the color property to set foreground colors for
elements. Remember that the element border defaults
to the element color unless you specifically state a
border color.
• Background colors affect any padding areas in the
element.They can be applied to both block-level and
inline elements.
• Choose background images that do not detract from
the legibility of your content.
• Use the background-repeat and background-position
properties to control the appearance of images in the
background.
• Test your work on different browsers and computing
platforms, as they render colors differently. Test at
different color depths as well.
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